Film appropriate for printing

ABSTRACT

The invention relates to printable films comprising a substrate and at least a surface layer, said layer covering at least one face of said substrate and comprising a water-dispersible polymer and an ethylenically unsaturated compound; to a process for the manufacture of such films; to printed films and especially to printed labels obtained from such printable films.

DESCRIPTION

[0001] The present invention relates to an improved printable filmhaving good ink adhesive properties and relates more particularly to animproved printable film having good adhesive properties when used withradiation curable ink.

[0002] In recent years, diversification of printed products has requiredprinting on a wider variety of materials in sheets; for example, papers,synthetic papers, polymer films such as thermoplastic resin films,metallic foils, metallized sheets, etc. These printed items are printedby methods such as by offset printing, gravure, flexography, screenprocess printing and letterpress printing. In these printing methods, amethod which uses radiation curable ink has recently become popularbecause radiation curable inks cure rapidly, and the printing methodwhich uses radiation curable ink is of superior handling. Radiationcurable inks are known to be useful in the printing of packaging, labelsand non absorbing printing materials. Radiation curable printing inkstypically contain unsaturated acrylates, polyesters, photoinitiators,and additives. In electron beam cured inks however, the photoinitiatorsmay be omitted.

[0003] After deposition of the radiation curable ink on the printableitem, the print is exposed to radiation and hardens within a fraction ofa second. Printing speeds up to 300 m/min are attained during continuousprinting. At present, there is a great demand for sheet-like printableitems.

[0004] In printing methods, the printing sheet requires sheet runningproperties, anti-blocking properties, producing uniform spread of theink over the surface of the sheet, as well as antistatic properties.Besides these generally required properties, in printing methods whichuse radiation curable ink, the printing sheet requires in particular theproperty of adhering strongly to radiation cured ink.

[0005] In particular, radiation curable ink printed polymer films,intended for use as labels, for example in the bottle labelling market,should be resistant to both freezing water conditions (i.e. storage incoolers or ice buckets for.24 hours) and boiling water conditions (i.e.pasteurization by immersion in water at 95° C. for up to 1 hour).

[0006] European patent application EP-A1-410051 discloses printingsheets comprising a support layer and a surface layer on at least oneface of said support, said surface layer containing at least an acrylatebased polymer and an unsaturated compound (cinamic acid or derivativesthereof.

[0007] Neither this document, nor any other document of the state of theart do teach anything about the possibility of use of other monomers toreplace cinamic acid.

[0008] However, the above listed materials formed in sheets, especiallypolymer films do not sufficiently adhere to radiation curable ink afterprinting and curing, especially in these extreme conditions. Accordinglythe printed and radiation cured ink has a problem in the fact that theprinted and radiation cured ink separates from the polymer film.

[0009] Therefore, a printable film which has superior adhesion to theradiation curable ink, even in extreme conditions, is required.

[0010] The present invention has the above-described problems in mind,and an object of this invention is therefore to provide a printablefilm, particularly a printable polymer film, which has the superiorproperties required of printable films, and particularly has superioradhesive properties with radiation cured ink.

[0011] Accordingly, the present invention, provides a printable filmcomprising a substrate and at least a surface layer which covers atleast one face of the substrate and which comprises a water dispersiblepolymer and an ethylenically unsaturated compound.

[0012] Suitable substrates, which can be used in this invention, arepolymer films, especially polyolefin films, papers, synthetic papers,woven fabrics, nonwoven fabrics, ceramic sheets, metallic plates, andmultilayer composite sheets formed by combination of said materials. Forprintable film intended for use as labels, polyolefin films arepreferred, especially oriented polypropylene films, and still morepreferred is an oriented polypropylene film according to the EuropeanPatent Application 202812.

[0013] In the present specification, the expression “printable film”denotes a film which is capable to be directly inked, i.e. a film ofwhich the surface layer is strong enough to resist the pull of the tackyink, otherwise areas of the surface layer may be pulled away from thesurface, giving a defect known as picking.

[0014] The expression “at least a surface layer” denotes a layer whichcovers at least one face of the substrate listed above. This surfacelayer comprises a water dispersible polymer, for example a waterdispersible acrylic or urethane polymer. In the present specification,an “acrylic polymer” means a (co)polymer obtained by the free-radicaladdition polymerization of at least one (meth)acrylic type monomer andoptionally of other vinylic or allylic compounds. It is essential thatthis acrylic polymer should be able to provide a smooth film-formed andreasonably open surface.

[0015] A wide variety of acrylic polymers are able to fulfill thisrequirement. Suitable acrylic polymers are homopolymers of (meth)acrylicacid or alkyl (meth)acrylate, the alkyl radical having 1 to 10 carbonatom, or copolymers of two or more of the said (meth)acrylic typemonomer and optionally of other vinylic or allylic compounds.

[0016] As said above, a water dispersible urethane polymer may alsosuitably be used. As with the acrylic polymer, it is essential that thisurethane polymer should be able to provide a smooth film-formed andreasonably open surface.

[0017] A wide variety of urethane polymers are able to fulfill thisrequirement. Suitable urethane polymers are for example the reactionproduct of an isocyanate-terminated polyurethane prepolymer formed byreacting at least an excess of an organic polyisocyanate, an organiccompound containing at least two isocyanate-reactive groups and anisocyanate-reactive compound containing anionic salt functional groups(or acid groups which may be subsequently converted to such anionic saltgroups) or non-ionic groups and an active hydrogen-containing chainextender.

[0018] The surface layer comprises also an ethylenically unsaturatedcompound.

[0019] The ethylenically unsaturated compound is selected to be misciblein the wet stage in the aqueous phase and to be compatible in the drystage with the water dispersible polymer itself. Consequently, theethylenically unsaturated compound acts as a plasticiser for the surfacelayer once hardened allowing the easy penetration of the radiationcurable ink thereto.

[0020] The ethylenically unsaturated compound must also be able, whenthe printed film is submitted to radiations in order to cure the ink, toreact with the unsaturated components of the ink which has penetratedinto the surface layer.

[0021] This reaction between the ethylenically unsaturated compounds ofthe surface layer and the unsaturated compounds of the radiation curableink forms chemical bonds between those compounds and simultaneouslycrosslinks the surface layer, thereby generating the final resistantproduct.

[0022] Preferably, the ethylenically unsaturated compound contains 1 to10 ethylenical bonds per molecule and still more preferably 2 to 5ethylenical bonds per molecule.

[0023] Suitable ethylenically unsaturated compounds are the esterderivatives of α, β-ethylenically unsaturated acids, such as acrylic ormethacrylic acids, itaconic or citraconic acids, maleic or fumaricacids, etc. with polyols or alkoxylated polyols.

[0024] The suitable polyols include saturated aliphatic diols such asethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, dipropylene glycol, tripropylene glycol, butylene glycols,neopentyl glycol, 1,3- and 1,4-butane diols, 1,5-pentane diol,1,6-hexanediol and 2-methyl-1,3 propanediol. Glycerol,1,1,1-trimethylolpropane, bisphenol A and its hydrogenated derivativesmay also be used. The suitable alkoxylated polyols include theethoxylated or propoxylated derivatives of the polyols listed above.

[0025] Examples of ethylenically unsaturated compounds which can be usedaccording to the invention are polyfunctional acrylates such asdifunctional acrylates, such as 1,4-butane diol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, triethylene glycoldiacrylate, polyethylene glycol diacrylate, tripropylene glycoldiacrylate, 2,2-dionol diacrylate, bisphenol A diacrylate, etc.,trifunctional acrylates such as pentaerythritol triacrylate,trimethylolpropane triacrylate, etc., tetrafunctional acrylates, etc.

[0026] It is to be understood that the methacrylate derivativescorresponding to these acrylate derivatives could also be used.

[0027] Moreover, it has been found that polyallyl derivatives such astetraallyloxyethane are also suitable. Preferably ethoxylatedtrimethylolpropane triacrylate (EBECRYL 1160 from UCB CHEMICALS) isused.

[0028] The amount of the ethylenically unsaturated compound can be offrom about 2 to about 90% by weight of the acrylic polymer, andpreferably is from about 2 to about 15% (in the present specification,all percentages are dry weight based).

[0029] In order to improve the hardness and/or water resistance of thesurface layer deposited on the film and consequently, of the finishedproduct, a crosslinking agent may advantageously be added to the surfacelayer. However, it is to be noted that such a crosslinking agent shouldbe chosen so as the surface layer, once hardened, allows the easypenetration of the radiation curable ink thereto.

[0030] Coordinating metal ligands which can form stable coordinatedstructures with carboxy or carbonyl functionality perfectly fulfill thisrequirement. Ammonium zirconium carbonate (stabilised or not) isparticularly preferred. The amount of the crosslinking agent can be upto 5% by weight of the acrylic polymer, and preferably is from 1 to 5%by weight of the acrylic polymer.

[0031] The surface layer can contain all other additional agents, ifnecessary, for preventing the blocking of one sheet to another, and forimproving the sheet running property, antistatic property,nontransparency property, etc. These additional agents are generallyadded in a total amount not exceeding about 40% by weight of the acrylicpolymer. As said additional agent, for example, a pigment such aspolyethylene oxide, silica, silica gel, clay, talc, diatomaceous earth,calcium carbonate, calcium sulfate, barium sulfate, aluminium silicate,synthetic zeolite, alumina, zinc oxide, titanium oxide, lithopone, satinwhite, etc. and cationic, anionic and nonionic antistatic agents, etc.may be used.

[0032] According to the present invention, the surface layer may beapplied as an aqueous dispersion at about 0.5 to about 2.5 g/m² on thesubstrate by the method of roll coating, blade coating, spray coating,air knife coating, rod bar coating, reverse gravure, etc. on thesubstrate and then dried, for example, in a hot air oven.

[0033] After the drying step, the surface layer comprises thus the waterdispersible polymer, smoothly crosslinked by the crosslinking agent and,included in the acrylic polymer matrix, the ethylenically unsaturatedcompound.

[0034] As said above, this allows the easy penetration of the radiationcurable ink into the surface layer as well as its subsequent reactionwith the ethylenically unsaturated compound.

[0035] Before applying the surface layer, the surface of the substratecan be first pretreated in a conventional manner with a view to improveits adhesiveness. For this purpose, it is possible, for example, topretreat the substrate by the corona effect but it should be understoodthat all known techniques aiming at improving the surface of asheet-like item with a view of the application of a composition, may besuitable.

[0036] It has been observed that in certain cases, the surface layercontemplated herein do not adhere well to film surfaces even when thelater have been subjected to well-known pretreatment operations such as,for example, treatment by corona discharge, flame, or oxidizingchemicals. It has been found, however, that the use of primersintermediate between the substrate and the surface layer provides a highlevel of adherence.

[0037] Indeed, in applying coating layer, to a polyolefin film substrateit is generally desirable first to apply an intermediate primer oranchor coating layer to the substrate to ensure adequate adhesionbetween the substrate and subsequently applied coating layer. Although afilm according to the invention may be utilized as such a substrate perse (for example, a nonprimed printable film according to the inventionis able to resist the freezing water conditions), it is preferred toutilise as a substrate for a subsequently applied layer a film to thereceptive surface(s) of which has been applied a primer or anchorcoating layer.

[0038] Advantageously, the film comprises thus further a primer layerbetween the substrate and the surface layer(s).

[0039] As examples of suitable primers, polyethylene imine orpolyurethane acrylate primers crosslinked by isocyanate, epoxy,aziridine or silane derivatives may be cited.

[0040] The primer resin may be applied by conventional coatingtechniques—for example, by a gravure roll coating method. The resin isconveniently applied as a dispersion or as a solution. Economically itwould be preferable to apply the resin as a dispersion in water. Aqueousdispersion techniques have the added advantage that there is no residualodour due to the solvent present which is generally the case when anorganic solvent is used. However, when using aqueous techniques it isusually necessary to heat the film a higher temperature to dry off thedispersant than with systems using an organic solvent or dispersant.Furthermore, the presence of a surfactant, which is generally used toimprove the dispersion of the coating in water, tend to reduced theadhesion between the resin and the base film. Thus, it is also possibleto apply the resin from an organic solvent or dispersant. Examples ofsuitable organic solvent include alcohols, aromatic hydrocarbonsolvents, such as xylene, or mixtures of such solvents as isappropriate.

[0041] A reverse face of the substrate, namely a face not covered by thesurface layer, can be covered with a pressure-sensitive adhesive layerwhich consists of a commonly used pressure-sensitive adhesive agent.Furthermore, if necessary, a releasing film or sheet consisting of areleasing agent, can cover the pressure-sensitive adhesive layer. Thislaminate comprising the printing sheet according to the invention can beused as an adhesive label which may be affixed to most types of surface.

[0042] Another aspect of the invention concerns thus a printable filmintended for labels comprising a substrate of which only one face iscoated with a surface layer and of which the other face is coated with apressure-sensitive adhesive layer which is itself covered with areleasing film or sheet.

[0043] Another aspect of the present invention relates to a process forthe manufacture of a printable film comprising the step of coating atleast one side of a substrate with an aqueous dispersion comprising awater dispersible polymer and an ethylenically unsaturated compound andoptionally a crosslinking agent and/or conventional additives, andcomprising further the step of drying the coating so obtained.

[0044] Advantageously, the process of manufacture of a printable filmcomprises before the step of coating at least one face of the substrate,a further pretreatment step of the substrate (such as corona dischargetreatment) and still more advantageously a step of priming of thesubstrate with a primer.

[0045] In a particular embodiment directed to the preparation of labels,only one face of said substrate is coated with a surface layer and theone face-coated substrate so obtained is coated with apressure-sensitive adhesive or in a variant, the pressure-sensitiveadhesive may be transferred from a release liner with which the coatedsubstrate is combined.

[0046] Another object of the present inventions concerns a printed filmcomprising a substrate of which at least one face is coated with asurface layer comprising a water dispersible polymer and anethylenically unsaturated compound, said coated face of the substratebeing printed by conventional methods such as offset printing, gravure,flexography, screen process printing and letterpress printing usingradiation curable ink and subsequently radiation cured.

[0047] Ink formulations for radiation curing contains generallypigments, vehicle, solvent and additives. The solvents in these systemsare low-viscosity monomers, capable of reacting themselves (i.e., usedas reactive diluents). The vehicle is usually composed of a resinderived from unsaturated monomers, prepolymers or oligomers such asacrylates derivatives which are able to react with the ethylenicallyunsaturated compound of the surface layer. For a UV ink, the “additives”contain a large amount of photoinitiators which respond to the photonsof UV light to start the system reacting.

[0048] A UV ink formulation may be generalized as: pigment 15-20%prepolymers 20-35% vehicle 10-25% photoinitiators  2-10% other additives 1-5%.

[0049] For an electron beam curable ink, the “additives” containgenerally no photoinitiator.

[0050] The low viscosity monomers, sometimes termed diluents, arecapable of chemical reactions which result in their becoming fullyincorporated into the ultimate polymer matrix.

[0051] The vehicle provides the “hard resin” portion of the formulation.Typically, these are derived from synthetic resins such as for example,urethanes, epoxides, polyesters which have been modified by reactionwith compounds bearing ethylenic groups such as for instance(meth)acrylic acid, hydroxyethyl(meth)acrylate, reaction product ofcaprolactone with unsaturated compounds bearing a hydroxyl group, andthe like.

[0052] Appropriate adjustments could be made in the selection of theprepolymers and monomers used in order to achieve the requiredviscosities for the different methods of application.

[0053] Another aspect of the invention relates to a process for themanufacture of a printed film comprising the steps of

[0054] a) coating a substrate with an aqueous dispersion comprising awater dispersible polymer and an ethylenically unsaturated compound;

[0055] b) drying the coating so obtained;

[0056] c) inking the dried coating with radiation-curable ink;

[0057] d) curing the ink with UV or EB radiations.

[0058] It is to be noted that the different steps of this process may beeffected in the same conditions (speed, costs, . . . ) as withconventional surface layer.

[0059] Finally, the invention concerns also a printed film as obtainedby inking a printable film according to the invention, and especially aprinted label so obtained.

[0060] The following Examples are given for the purpose of illustratingthe present invention.

[0061] In these Examples, the determination of certain characteristicvalues was carried out in accordance with the methods described below.

[0062] The adhesion of the radiation cured ink to the film in boilingwater conditions (simulating pasteurization) is controlled according tothe following procedures.

[0063] A) A stirred waterbath is heated to 95° C. Once the temperatureremains stable, the sample of the printed film to be tested is immersedinto the water. After 45 minutes, the sample is removed from thewaterbath and scratched with moderate pressure with a coin held squareto the surface of the sample. The loss of print is reported as “pass” or“fail” wherein “pass” means no loss of print and “fail” means noticeableloss of print. B) The sample is removed from the waterbath at 95° C.after 45 minutes as described in A.

[0064] An adhesive tape is applied on the sample and then the adhesivetape is quickly removed. In this way, the properties of the ink which isprinted on the printing sheet are observed.

[0065] The surface percentage of ink removed (visual estimation) isreported.

[0066] The adhesion of the radiation cured ink to the film in freezingwater conditions (simulating ice chest immersion) is controlledaccording to the following procedure.

[0067] A waterbath containing a mixture of ice and water (50:50) iscooled to 0° C. Once the temperature remains stable, the sample of theprinted film to be tested is immersed into the water.

[0068] After 24 hours, the sample is removed from the waterbath andscratched with moderate pressure with a coin held square to the surfaceof the sample. The loss of print is reported as “pass” or “fail” in thesame manner as described above.

EXAMPLE 1

[0069] A corona discharge treated polypropylene film of 50 mm thicknessis primed on one face at 0.2 g/m² with a polyurethane acrylate to whichan isocyanate crosslinker is added prior to coatings. The primed face ofthe substrate is subsequently overcoated, at 1.0 g/m² and by reversegravure, with an aqueous dispersion containing 21.0 kg of an aliphaticpolyester based polyurethane (DAOTAN VTW 1238 from HOECHST; solidscontent 50%) which represents 80.8% (dry weight) of the aqueousdispersion, 0.9 kg of tripropylene glycol diacrylate (solids content100%; 7% (dry weight) of the aqueous dispersion), 1.1 kg of ammoniumzirconium carbonate (solids content 20%; 1.7% (dry weight) of theaqueous dispersion), 4.3 kg of colloidal silica (LUDOX HS40 from DUPONT; solid content 30%; 10% (dry weight) of the aqueous dispersion) and0.65 kg of silica gel as antiblocking agent (GASIL HP 250 fromCROSFIELD; solids content 10%; 0.5% (dry weight) of the aqueousdispersion).

[0070] The coated film is then dried in a hot air oven.

[0071] The coated film so obtained is then printed in a screen printingprocess with RSP series ink (from NORCOTE), at 12 g/m².

[0072] The printed film so obtained is then UV cured with a mediumpressure mercury vapour lamp (120 W/cm) at 12.2 m/min.

[0073] The printed film finally obtained is tested according to themethod given above. The results are reported in Table 1.

Comparative Example A

[0074] A corona discharge treated polypropylene film (but not primed) of50 mm thickness, is printed and then cured as described in Example 1.

[0075] The printed film finally obtained is tested according to themethod given above. The results are reported in Table 1.

Comparative Example B

[0076] A printed film is prepared exactly as described, at Example 1,except that the aqueous dispersion does not contain tripropylene glycoldiacrylate.

[0077] The printed film finally obtained is tested according to themethod given above. The results are reported in Table 1. TABLE 1Freezing water Boiling water condition condition Example Scratch test %ink removal Scratch test 1 Pass  0% Pass A* Fail 95% Fail B* Fail 60%Fail

[0078] These results show that the printable film according to theinvention produces highly desirable properties with respect to adhesionof the radiation curable ink to the substrate. Especially, the influenceof the presence of the surface layer may be noted from the comparisonbetween examples A and B and the influence of the ethylenicallyunsaturated compound may be noted from the comparison between examples Band 1.

EXAMPLE 2

[0079] A printed film is prepared exactly as described at example 1,except that the ethylenically unsaturated compound is replaced withtriacrylate of ethoxylated trimethylpropane (EBECRYL 1160 from UCB,S.A.).

EXAMPLE 3

[0080] A printed film is prepared exactly as described at example 1,except that the urethane polymer is replaced with an anionic acrylicpolymer (NEOCRYL XK-90 from ZENECA).

EXAMPLE 4 to 6

[0081] Printed films are prepared exactly as described at example 2,except that the urethane polymer is replaced respectively with ananionic acrylic polymer (NEOCRYL XK-90 from ZENECA) (example 4), with astyrene acrylic copolymer (GLASCOL LE 31 from ALLIED COLLOIDS) (example5) and with another anionic acrylic polymer (NEOTAC A-572 from ZENECA)(example 6).

[0082] The printed films obtained at examples 2 to 6 are testedaccording to the method given above and the results are reported inTable 2. TABLE 2 Freezing water Boiling water condition conditionExample Scratch test % ink removal Scratch test 2 Pass 0 Pass 3 Pass 0Pass 4 Pass 0 Pass 5 Pass 0 Pass 6 Pass 0 Pass

[0083] These results show that a wide variety of surface layer andethylenically unsaturated compounds may be used.

1. Printable film comprising a substrate and at least a surface layer,said layer covering at least one face of said substrate and consistingessentially of 10 to 98% in weight of a water-dispersible polymer ableto provide a smooth-film formed and reasonably open surface and 2 to 90%in weight of an ethylenically unsaturated compound selected frompolyfunctional acrylates resulting from the esterification of a polyolwith (meth)acrylic acid or polyallyl derivatives, whereby said surfacelayer does not contain an addition polymerisation photoinitiator. 2.Printable film according to claim 1, characterised in that the substrateis selected from the group consisting of polymer films, polyolefinfilms, papers, synthetic papers, woven fabrics, nonwoven fabrics,ceramic sheets, metallic fiber sheets, metallized sheets (film),metallic foils, metallic plates, and multilayer composite sheets formedby combination of said materials.
 3. Printable film according to claim2, characterised in that the substrate is an oriented polypropylenefilm.
 4. Printable film according to anyone of claims 1 to 3,characterised in that the water dispersible polymer consists essentiallyof a homopolymer of (meth)acrylic acid or alkyl (meth)acrylate, thealkyl radical having 1 to 10 carbon atom, or a copolymer of two or moreof said monomers or optionally of other vinylic or allylic compounds. 5.Printable film according to anyone of claims 1 to 3, characterised inthat the water dispersible polymer is a urethane or urethane acrylatepolymer.
 6. Printable film according to anyone of claims 1 to 5,characterised in that said surface layer comprises further acrosslinking agent in an amount of from 1 to 5% by weight of the waterdispersible polymer.
 7. Printable film according to anyone of claims 1to 6, further comprising, between said substrate and said surfacelayer(s), a primer layer.
 8. Printable film according to anyone ofclaims 1 to 7, characterised in that only one face is coated with asurface layer and in that the reverse face of the substrate is coveredwith a pressure-sensitive adhesive layer.
 9. Process for the manufactureof a printable film according to anyone of claims 1 to 8, comprising thestep of coating at least one side of the substrate with an aqueousdispersion comprising the water dispersible polymer and theethylenically unsaturated compound and comprising further the step ofdrying the coating so obtained.
 10. Process according to claim 9,characterised in that the aqueous dispersion comprises further acrosslinking agent.
 11. Process according to anyone of claims 9 or 10,characterised in that it comprises a further step of priming of thesubstrate, before applying said aqueous dispersion.
 12. Process for themanufacture of a printed film comprising the steps of a) coating asubstrate with an aqueous dispersion comprising a water dispersiblepolymer able to provide a smooth-film formed and reasonably open surfaceand an ethylenically unsaturated compound selected from polyfunctionalacrylates resulting from the esterification of a polyol with(meth)acrylic acid or polyallyl derivatives, whereby said surface layerdoes not contain an addition polymerisation photoinitiator; b) dryingthe coating so obtained; c) inking the dried coating with a radiationcurable ink; d) curing the ink with UV or EB radiations.
 13. Printedfilm obtained by inking a printable film according to anyone of claim 1to 8 by means of a radiation curable ink and by curing the said ink bymeans of radiations.
 14. Label obtained by inking of a printable filmaccording to claim 8, with a radiation curable ink and by curing of saidink with radiations, said film being combined before or after printingwith a pressure sensitive adhesive layer and to a release film. 15.Container, in particular bottle, labelled with a label according toclaim 14.